The present invention is directed to bicycles and, more particularly, to a shift control apparatus for a bicycle transmission.
Bicycles often are equipped with transmissions that provide a plurality of gear ratios to facilitate riding over various terrain. Such transmissions typically comprise either internal or external transmissions. Internal transmissions usually comprise a planetary gear mechanism disposed within the rear wheel hub. External transmissions typically comprise a plurality of sprockets that rotate coaxially with the rear wheel, and a derailleur that switches the drive chain among the plurality of sprockets. In either case, the transmission usually is connected to a shift control lever mounted to the handlebar or to some other convenient location. The rider then may select a desired gear ratio by manually manipulating the shift control lever.
Recently, automatic shift control devices have been developed so that a proper gear ratio may be selected without requiring manual control by the rider. The automatic shift control device typically selects the proper gear ratio based on bicycle speed or crank RPM. When sensing bicycle speed or crank RPM, a magnet is mounted to the bicycle wheel or crank arm, and a reed switch is mounted to the bicycle frame to sense the passage of the magnet and provide a signal pulse upon each revolution of the wheel or crank arm. Bicycle speed may be calculated from the interval between pulses and the diameter of the wheel. The controller for the automatic shift control device establishes an upshift threshold value and a downshift threshold value based on bicycle speed. The controller provides a signal for upshifting the bicycle transmission to the next higher gear ratio when the bicycle speed rises above the upshift threshold value, and the controller provides a signal for downshifting the bicycle transmission to the next lower gear ratio when the bicycle speed falls below the downshift threshold value. The upshift threshold value usually is set to a value slightly higher than the downshift threshold value of the next highest transmission gear. This produces a hysteresis effect so that the bicycle transmission is not rapidly shifted back and forth between adjacent gears, known as chattering, when the bicycle speed hovers near the upshift and downshift threshold values.
Chattering usually is easy to prevent using a hysteresis effect when bicycle speed is calculated from signal pulses that occur at a relatively low frequency of about once per wheel revolution. However, when the frequency of signal pulses is increased by disposing a plurality of magnets circumferentially around the wheel, there is a risk that temporary fluctuations in bicycle speed will cause undesired shifting of the bicycle transmission. For example, when climbing an uphill stretch, bicycle speed may momentarily fluctuate close to the upshift threshold value for a very brief time, thus resulting in unintended upshifting of the bicycle transmission followed by downshifting immediately thereafter. Such unintended shift operations require the rider to frequently change pedaling force in order to maintain the desired speed, thus creating the risk of conflicting with the rider's pedaling rhythm as well as unstable bicycle operation.
To avoid such undesirable operation, automatic shift control devices have been developed wherein upshifting will not occur unless a plurality of signal pulses indicate that the bicycle speed is above the upshift threshold value. Such a device is disclosed in Japanese Unexamined Patent Application No. 2003-120799. In that device, all speed signal pulses that occur during a predetermined time interval must indicate that the bicycle speed is above the upshift threshold value. If so, then upshifting will occur after the predetermined time interval. However, if even one speed signal that occurs during the predetermined time interval indicates that the bicycle speed is below the upshift threshold value, then upshifting will not occur. The net result is that the automatic shift control device will upshift the bicycle transmission only when riding conditions exceed the upshift threshold value for a predetermined time interval, and many instances of unintended upshifting of the bicycle transmission can be prevented even when riding conditions are sensed frequently.
Unfortunately, even with automatic shift control devices that require upshift threshold values to be exceeded for a predetermined time period, there are still instances when unintended upshifting may occur. For example, if the bicycle is descending a slope on a sidewalk at a parking lot exit, accelerating on an uphill stretch by standing up while pedaling, or suddenly accelerating on a flat road, all of the sensor results within the predetermined time interval may indicate that the bicycle speed exceeds the upshift threshold value. However, the resulting upshift may cause the rider to experience discomfort due to a sharp increase in the effort needed to pedal.